Friction coupling devices and fluid coupling devices that drive radiator cooling fans for over the road trucks, such as class 8 trucks, are generally of two types, dry friction clutch assemblies and viscous drives, respectively.
Dry friction clutch assemblies tend to have two operating conditions “ON and OFF” referring to when a friction clutch is either fully engaged or fully disengaged. When a friction clutch assembly is providing cooling the clutch is fully engaged and not slipping. When the friction clutch assembly is not providing cooling the assembly is fully disengaged and slip speed is at a maximum between a clutch plate and an engagement surface.
Dry friction clutch assemblies generally have low thermal capacity since they typically do not incorporate fluid flow cooling mechanisms. Therefore these clutch assemblies have minimal cooling capability and are unable to cycle repeat in short durations of time. The thermal energy that is generated during engagement at high engine speeds can cause the clutch lining to “burn up” or cause the clutch assembly to become inoperative.
Viscous drives, on the other hand, have become popular due to their ability to cycle repeat, engage at higher engine speeds, and operate at varying degrees of engagement. Viscous drives have an operating range of engagement and are generally less engaged at higher engine speeds and generally more engaged at lower engine speeds. Viscous drives never fully engage due to the torque transfer through viscous fluid shear.
Due to the size constraints, viscous drives are also thermally and torsionally limited since viscous drives are always slipping to some degree, they are incapable of turning at fully engaged peak operating speeds. Furthermore, the continuous slipping means viscous drives are continuously generating heat, unlike friction clutch assemblies. Viscous drives are further limited in that as engine cooling requirements increase, larger and more costly viscous drives are required. Thus, some high cooling requirement vehicles viscous drives can become impractical in size and cost.
Due to increasing engine cooling requirements, it is desirable that a fan drive system be capable of not only providing increased cooling over traditional fan drive systems, but also that it have the combined advantages of a friction clutch assembly and of a viscous drive, as stated above, without the associated disadvantages. It is also desirable that the fan drive system be practical and reasonable in size and cost and to be approximately similar to and preferably not to exceed that of traditional fan drive systems.
To overcome the disadvantages of the aforementioned traditional fan drive systems, a new fan drive system has been developed which can be referred to as a solenoid actuated hydraulically controlled fan drive system. A housing assembly is provided which is typically 12-16 inches in diameter. To minimize parasitic drag losses, the housing is not completely filled with hydraulic fluid, but is typically filled such that there is 1-2 inches of hydraulic fluid spaced around a circumference (assuming that the housing is being spun). The fan drive system is engine driven via a belt or chain driven pulley. A stationary bracket rotatably mounts the pulley to the chassis of the vehicle. The pulley is fixably connected to the housing assembly. A clutch assembly within the housing assembly is selectively engaged to connect the rotative fan with the housing assembly. The hydraulic aforementioned clutch is activated via hydraulic pressure. The hydraulic pressure is generated through the use of a pitot tube. The pitot tube is fixably connected to the mounting bracket. The fluid, which is rotating within the housing, is used to generate pressure through momentum exchanged at an aperture in the stationary pitot tube. The pitot tube is also fluidly connected with a piston engaging circuit through which a clutch friction pack engages a fan hub which is rotatably mounted to the housing assembly. To control the amount of fan engagement with the housing assembly via the friction pack, a hydraulic control arrangement is provided. The hydraulic control arrangement regulates the pressure within the piston housing by selectively connecting the pitot tube with a reservoir sump. The reservoir sump occurs due to the void of fluid in the center of the housing assembly. A solenoid actuated relief valve is utilized to selectively regulate the fluid connection between the pitot tube and the low pressure sump formed within the housing assembly. To ensure full engagement of the rotating fan hub with the housing (fan locked in position), the solenoid actuated relief valve completely blocks the sump, causing the full pressure developed by the pitot tube to be applied to the friction pack, which torsionally connects the fan hub with the housing assembly. The amount of torsional connection between the housing and fan hub is varied by utilizing an electronic controller system to selectively open and close the solenoid valve, thereby controlling the amount of pressure applied to the friction pack by the piston.
Since the pressure acting on the piston is controlled by the solenoid, operation of the fan drive system during a period of solenoid or electrical failure must be considered. In most applications, “fail safe” operation provided by a bias spring in the valve that is overcome by the solenoid. In an instance of electrical failure, the spring will close the relief valve, providing full pressure from the pitot tube to the piston. The full pressure will ensure that the fan hub will always be engaged on. However, there are major disadvantages of a failure mode operation wherein the fan is fully engaged. The fully engaged failure mode causes very heavy loads to be placed on the fan even though full cooling capacity is not required. Furthermore, a fully engaged failure mode causes unnecessary fuel consumption and can cause damage to the transmission and accessory belt drive system. Conversely, if the failure mode is such that the fan is permanently disengaged, then the vehicle will not have adequate cooling during most operating conditions. Therefore, it is desirable to provide a failure mode operation for the hydraulically controlled fan drive system which overcomes the aforementioned disadvantages.